The present invention relates to suspension devices and methods, and in particular to structures for mounting force-versus-displacement sensors, whereby external stress sources are isolated from active sensor components.
Accelerometers generally measure acceleration forces applied to a body by being mounted directly onto a surface of the accelerated body. One common type of accelerometer employs one or more force-versus-displacement or xe2x80x9cforce/displacementxe2x80x9d sensors for measurement of acceleration. Accelerometers employing two force/displacement sensors instead of the necessary minimum one sensor gain considerable advantage. If the two sensors operate in a push-pull mode, then many error sources such as thermally driven effects or drift may be rejected as common mode, while the difference signal represents the desired acceleration measurement. Occasionally, designs using two force/displacement sensors include two completely separate proof masses, which results in essentially two accelerometers, each having its own sensor, but operating in opposite directions. For numerous reasons, however, a two proof mass solution is not preferred. Rather, it is generally advantageous to have only one proof mass in an accelerometer.
A typical example of a prior art micromachined two-sensor/single proof mass accelerometer, commonly referred to as a Rectangle design, is illustrated in FIGS. 1A and 1B. The accelerometer 10 illustrated in FIGS. 1A and 1B is a miniature structure fabricated from a substrate 12 of semiconductor material by conventional micromachining techniques. The substrate 12 is formed of a monocrystalline silicon material in a substantially planar structure, i.e., having substantially planar and parallel opposing offset upper and lower surfaces. The silicon substrate 12 often includes an upper silicon or active layer 14 that is electrically isolated from an underlying substrate 16 by an insulating layer 18, or an insulating layer is applied to active layer 14, as shown and described in U.S. Pat. No. 5,948,981, entitled Vibrating Beam Accelerometer, issued on Sep. 7, 1999, and assigned to the Assignee of the present application, the entirety of which is incorporated herein by reference. The insulating layer 18 is may be a thin layer, e.g., about 0.1 to 10.0 micrometers, of an oxide, such as silicon oxide. The silicon substrate 12 is usually formed by oxidizing active layer 14 and underlying substrate 16, and adhering the two layers together. A portion of active layer 14 may be removed to bring the layer 14 to the desired thickness. The silicon oxide layer 18 retains its insulating properties over a wide temperature range to ensure effective mechanical resonator performance at high operating temperatures on the order of 100 degrees Celsius. In addition, the insulating layer 18 inhibits undesirable etching of the active layer 14 during manufacturing.
The micromachined accelerometer 10 includes an acceleration sensor mechanism 20 having one or more flexures 22 pliantly suspending a proof mass 24 from an inner sensor frame or plate 26 for movement of the proof mass 24 along an input axis I normal to the proof mass 24. The flexures 22 are preferably etched near or at the center of the underlying substrate 16, i.e., substantially centered between the opposing upper and lower surfaces of the underlying substrate 16. Optionally, the flexures 22 are formed by anistropically etching in a suitable etchant, such as potassium hydroxide (KOH). The flexures 22 define a hinge axis H about which the proof mass 24 moves in response to an applied force, such as the acceleration of the accelerated body, for example, a vehicle, aircraft or other moving body having the accelerometer 10 mounted thereon. The sensor mechanism 20 includes a pair of force/displacement sensors 28 coupled between the proof mass 24 and the sensor frame 26 for measuring forces applied to the proof mass 24. The force/displacement sensors 28 are, for example, mechanical resonators formed from the active silicon layer 14 as double-ended tuning fork (DETF) force sensors.
A known oscillator circuit, shown in FIG. 1C and described in above-incorporated U.S. Pat. No. 5,948,981, drives the mechanical resonators 28 at their resonance frequency. FIG. 1C illustrates a representative oscillation circuit 50 in which vibrating beams of the transducers 28 function as a resonator. A transimpedance amplifier 52 converts a sense current received from vibrating beams to a voltage. This voltage is filtered by a bandpass filter 54, which reduces noise, and the voltage amplitude is controlled by an amplitude limiter 56. The resulting signal is combined with the output or DC bias voltage from a DC source 58 in a summing junction 60. The DC bias voltage generates a force between electrodes and the beams of the force/displacement sensors 28. The signal from amplitude limiter 56 modulates this force causing the beams of the transducers 28 to vibrate laterally at their resonant frequency. This lateral beam motion, in turn, generates the sense current. An output buffer 62 isolates the oscillator from external circuitry connected to an output 64 of oscillation circuit 50. The gain in oscillation circuit 50 sustains oscillation of the beams of the force/displacement sensors 28.
In response to an applied force, the proof mass 24 rotates about the hinge axis H, causing axial forces, either compressive or tensile, to be applied to the mechanical resonators 28. The axial forces change the frequency of vibration of the mechanical resonators 28, and the magnitude of this change serves as a measure of the applied force or acceleration. In other words, the force/displacement sensors 28 measure the applied acceleration force as a function of the displacement of the proof mass 24.
Undesirable external stresses and strains may be induced in the sensitive acceleration sensor mechanism 20 by, for example, mechanical coupling of the accelerometer sensor frame 26 to a silicon cover plate 30 which in turn is typically connected to a ceramic or metal mounting plate 32. Any strains occurring in the sensor frame 26 are transmitted not only to the proof mass 24, but through the proof mass 24 to the two DETF resonators 28. Since the only significant compliance in the system is the sensing DETF resonators 28 themselves, almost the entire strain appears as an error output from the DETF resonators 28. Thus, undesirable errors are generated in the DETF resonators 28 from inputs having nothing to do with the acceleration being measured. These errors can be quite large since the compliance through the DETF resonators 28 must be low to detect acceleration with sufficient accuracy to be useful in practical systems.
Strain isolation within the micro-machined accelerometers is thus of paramount importance for good performance, i.e., accuracy. Strain isolation separates the mechanism from stresses mechanically induced during fabrication and assembly, and thereby reduces variations in resonance within the beams of the two vibrating-beam force sensing portion of the accelerometer mechanism. Strain isolation also separates the mechanism from stresses externally induced by shock, vibration and temperature variation within the operating environment.
Many methods are known for isolating the sensitive acceleration sensor mechanism 20 from such undesirable stresses and strains. Typically, cantilever-style isolation is provided wherein the sensor frame 26 is suspended from a second outer or external frame portion 34 by flexures 36 formed by overlapping slots 38 and 40 through the substrate 12. The sensor frame 26 is thus able to move relative to the outer frame 34, as shown and described in U.S. Pat. No. 5,948,981, which is incorporated herein. Such isolation minimizes the distortion of the sensor frame 26, and thereby decreases the effects of external stresses and strains on the mechanical resonators 28.
FIG. 1B is a cross-section view taken through the micromachined accelerometer 10 along the resonators 28. As discussed above and shown in FIG. 1B, the proof mass 24 is free to rotate about the flexures 22 when subjected to acceleration along the input axis I according to the principle of Newton""s law, F=ma. This rotation is constrained by the action of two force/displacement sensors 28, shown as DETF resonators, positioned on a surface of the mechanism as shown. These two vibrating beam force sensors 28 provide push-pull variable frequency output signals since, when the proof mass 24 is displaced relative to the plane of the sensor mechanism 20, one DETF resonator 28 is under compression while the other is under tension. The difference between the two frequencies represents the measured acceleration. Common mode frequency shifts, on the other hand, are rejected as errors driven by unwanted sources such as temperature, mechanism stress, or drift. FIGS. 1A and 1B thus demonstrate the cantilever-style isolation provided by the prior art.
Prior art designs have effectively used the cantilever-style strain isolation, new applications continually reduce the space available for the accelerometer. New constraints are placed upon the space available within the accelerometer for strain isolation. These new space constraints do not permit the cantilever-style strain isolation of the prior art. Accelerometer designers are thus challenged in providing sufficient strain isolation within minimum spacing.
The present invention overcomes size limitations of the prior art for providing on-die strain isolation, which is critical to isolating the accelerometer mechanism from externally induced stresses and the resultant strains, including strains induced during fabrication and assembly, cover plate attachment, header mounting, and environmental conditions during operation. The H-Beam strain isolator of the invention minimizes the impact of strains induced exterior to the die. The H-Beam strain isolator also provides for a symmetric strain isolation system, which reduces nonlinear effects such as those caused by eccentricities. The H-Beam strain isolator also minimizes alignment rotation error caused by the strains.
According to one aspect of the invention, a suspension structure is provided having a first elongated flexure having first and second ends structured for connection to a support structure, and a second elongated flexure having first and second ends structured for connection to a structure to be isolated from the support structure. A portion of the second flexure intermediate the first and second ends thereof is interconnected to a portion of the first flexure intermediate the first and second ends thereof.
According to another aspect of the invention, the first and second flexures of the suspension structure are spaced apart and interconnected by an interconnecting structure connected between the intermediate portion of the first flexure and the intermediate portion of the second flexure.
According to another aspect of the invention, each of the first and second flexures of the suspension structure are formed in a substrate having substantially parallel opposing offset upper and lower surfaces, the first and second flexures being defined by a plurality of slots formed through the substrate between the upper and lower surfaces.
According to another aspect of the invention, when in an unconstrained condition, each of the first and second flexures of the suspension structure is substantially straight between its respective first and second ends. In the unconstrained condition the first and second flexures of the suspension structure may be spaced apart and substantially mutually parallel and have an interconnecting structure connected therebetween.
According to another aspect of the invention, the suspension structure of the invention may also include a structure to be suspended and a support structure at least partially surrounding the structure to be suspended and spaced away therefrom. The first and second ends of the first flexure are connected to the support structure, and the first and second of the second flexure are connected to the structure to be suspended, which may be an accelerometer sensor mechanism.
According to still other aspects of the invention, methods for suspending and isolating an acceleration apparatus are provided.